1. Field of the Invention
The present invention relates to a method of and an apparatus for detecting a wavelength in a liquid, a method of and an apparatus for evaluating a total amount of dissolved gases in a liquid, and a method of and an apparatus for controlling a total amount of dissolved gases in a liquid.
2. Description of Related Art
Ultrasonic cleaning gets a lot of attention and is widely used because it enhances a cleaning process, saves labor of the cleaning process, realizes high precision cleaning, secures uniformity of cleaning accuracy, and rationalizes cleaning work.
The ultrasonic cleaning creates cavitation in a cleaning liquid by ultrasonic waves and utilizes the cavitation as physical cleaning force. The magnitude of the cavitation is dependent on temperature and an amount of dissolved gases in the cleaning liquid. If the cleaning liquid contains a large amount of dissolved gases in form of, for example, bubbles, effect of the cavitation will deteriorate because the dissolved gases reflect ultrasonic waves.
To maintain accuracy of the ultrasonic cleaning, it is important that a total amount of dissolved gases in the cleaning liquid is evaluated. If the total amount of dissolved gases exceeds a predetermined or set value, the cleaning liquid must be degassed or replaced with new one.
It is relatively easy to measure an amount of dissolved oxygen. It is, however, very difficult to measure an amount of other dissolved gases such as carbon dioxide and it is impossible to evaluate a total amount of dissolved gases if the dissolved gases contain such unmeasurable gases.
To solve this problem, Japanese Unexamined Patent Application Publication No. H05-57256 discloses a dissolved gas concentration meter to measure an amount of dissolved gases such as nitrogen and carbon dioxide that are poorly reactive.
The related art oppositely arranges first and second diaphragms and places an ultrasonic emitter made of a piezoelectric element on the first diaphragm and an ultrasonic sensor made of an ultrasonic wave strength measuring piezoelectric element on the second diaphragm.
The ultrasonic emitter emits ultrasonic waves and the ultrasonic sensor detects a value corresponding to a sound pressure of the ultrasonic waves. According to the detected value and a graph that relates the sound pressure corresponding value to an amount of dissolved gases, the related art measures an amount of dissolved gases.
It is unclear, however, if the related art is capable of measuring a total amount of a mixture of several kinds of gases because the relational graph used by the related art only refers to nitrogen gas.
Namely, the related art has not yet led to evaluate and control a total amount of dissolved gases in a liquid.
In the meantime, it is known that in ideal water at 25° C., the velocity of ultrasonic waves (sonic velocity) is 1496 m/sec. If there are bubbles of about 4 ppm in the water, the sonic velocity decreases to about 1000 m/sec.
Such decrease of ultrasonic wave due to bubbles is mentioned in J. Saneyoshi, Bulletin of Tokyo Institute of Technology, Series B, 1953, No. 1, p. 1 and A. Mallock, Proc. Roy. Soc. London, 84, p. 391 (1911). When the velocity of ultrasonic waves changes, the frequency of the ultrasonic waves is unchanged but the wavelength thereof changes. Accordingly, detecting a change in the wavelength of ultrasonic waves may be useful to find a change in dissolved gases (bubbles) in a liquid.
It is difficult, however, to detect a wavelength in a liquid as an indication of dissolved gases (bubbles).